Electric translation system



Jan. 28, 1941.

ELECTRIC TRANSLATION SYS P. J. LARSEN iled Jan. 22, 1958 4 Sheets-Sheetl INVENTOR. @Qaflsen ATTORNEY.

Jan. 28, 1. P. J. LARSEN ELECTRIC TRANSLATION SYSTEM 4 Sheets-Sheet, 2

Filed Jan. 22, 1938 F LTER e R i EQUALIZER In M n m/ N 7 b INVENTOR aulrserl ATTORNEY.

Jan. 28, 1941. P. J. LARSEN ELECTRIC TRANSLATION SYSTEM 4 Sheets-Sheet 3Filed Jan. 22, 1938 FILTER e OR EQUALIZER f I F'I l-TE I? a? 5 EQ AL ZERFig .9.

IN VENTOR.

f2 g: 106L.

BY yaw! i Jbarsen %4/ fia ATTORNEY.

Patentecl Jan. 28, 1941 UNITED STATES PATENT OF'FLIEE Radio PatentsCorporation, New York, N. Y., a corporation of New York ApplicationJanuary 22, 1938, Serial No. 186,371

2 Claims.

The present inventionyrelates to Wave translation systems,moreparticularly to improvements in and amethod of translating oramplifying variable electric currents as used in the high frequency 5and low frequency arts such as in radio, carrier telephony, telephone,telegraph, talking picture, sound systems and the like. Morespecifically the invention herein is concerned with amplifiers havingcorrective wave translation networks for eliminating distortioncharacteristics of impressed input signal and/ or to obtain band passfrequency characteristics from normally broad fiat frequency esponseamplifiers and .to obtain other advanlages which will be furtherexplained in the specification.

In my copending application, Serial No. 183,154, filed January 3, 1938,I have described an a plifier with one or more inverse feedbackarrangements for stabilizingthe operation, preventing distortion and toobtain band pass or other response characteristics.

The object of the present invention is to obtain frequencydiscrimination by utilizing selective networks .or filters ofpredetermined frequency characteristic in a feed forward? or parallelpath to an amplifier from a lower toa higher amplification level thereofand superimposing the voltage derived therefrom upon the stage of higheramplification level of the amplifier to alter the .50 amplifier outputfrequency characteristic .or any other characteristic of desired shapein asimple manner substantially without affecting the over- .all gain ofthe amplifier such as is the case in inverse feedback system and withoutthe use of complicated or unstable circuits which may affect theotherwise stable characteristics of the amplifier.

Another. object is the provision of an amplifier having a predeterminedfrequency'response char- 40 acteristic or band width suitable foramplifying and/or translating low frequency, intermediate frequency, andhigh frequency signal bands of any desired frequency range or band widthand comprising .coupling and frequency discrimination solely in theauxiliary feed forward control path for obtaining the said results.

A further object is to combine low pass and high pass wavefilterelements as coupling networks in'an amplifier circuit in such a manneras 5 to obtain amplification of a desired frequency band ordiscrimination above or below a first predetermined cut-off frequencyand. to balance out or neutralize all the currents having frequenciesabove or below another predetermined frequency 5 thereby to obtain afrequency response characteristic of theband passer band suppressiontype positioned .at any desired point on the frequency scale extendingfrom zero frequencyto .thelow (audio) intermediate, and high frequencyranges employed in practice. *5 A still. further object is to combineequalizer networks having predetermined frequency characteristics ascoupling networks in an amplifier circuit in such a manner as to obtainadesired amplification or attenuation of predetermined 10 characteristicby balancing out or neutralizing distinct frequenciesor ranges or addingvoltages tothe input signal voltage, thereby to obtain a predeterminedover-all frequency response characteristic of the amplifier.

Another object is to provide equalizing networks in an amplifier system,the equalizer network or networks having characteristics similar to thedistortion characteristics of a transmission line or other transmissionsystem precedingthe am- 20 'plifier in such a manner that the voltagesderived from said equalizer neutralize .or-become cumulativetothecontrol voltage :of the amplifier, thereby correcting theresponse-thereof resulting in a :fiat

or any other predetermined total output Jfrequency-response of theamplifier.

These and further .objects .and :advantages of the invention will become.more apparent as the following detailed description proceedsstaken withreference to the accompanying drawings illustrating several embodimentsof the invention, and

wherein;

Figure l-shows a simple amplifying circuit having a band pass typefrequency amplifying or transmission characteristic constructed in ac-1'35 cordance with the invention,

Figure 2 shows the frequency response curves illustrative-of thefunction and operation of the circuit according toFigure 1,

Figure 3 is a further modification of a circuit according to theinvention embodying features such as twin-triode tubes to improve'theoverall efficiencyand operation,

Figure 4 is a diagram illustrating a typical equalizer network for Wavecorrection purposes,

Figure 5 shows a frequency response ,curve illustrative of the functionand operation of the equalizer network of the type shoWnin'Figure .4when employed in the preceding amplifier aircuits, Figures 1 and 3,

Figures 6 and 7 are further circuits showing modifications of theinvention adaptedto improve the overall gain of the amplifier,

Figures 8 and 9 show diagrams of typical band pass filters which may beutilized in conjunction with the preceding circuits,

Figs. 10a and 10b show frequency response curves illustrative of thefunction and operation of the filter of Figure 9 in conjunction with thecircuits according to the previous figures,

Figure 11 is a further modification of the invention for obtaining aband pass characteristic from an amplifier with a normally broad andfiat frequency characteristic by both neutralization of the undesiredfrequencies and strengthening of the desired frequencies according tothe invention,

Figures 12a-12e show frequency response curves illustrative of thefunction and operation of the circuit constructed in accordance with thecircuit of Figure 11.

Similar reference characters identify similar parts throughout thedifferent views of the drawings.

Referring to Figure 1, wherein is shown a simple amplifying circuithaving a' predetermined frequency characteristic of the band pass typepositioned at any desired point of the frequency scale including theaudio, intermediate and high frequency ranges, numeral l represents anamplifying valve of standard construction, in the example shown, a valveof the pentode type having a cathode, an input or control grid 2, ascreen grid 3, a suppressor grid 4, and a plate or anode 5. Thesuppressor grid 4 is directly internally connected with the cathode in amanner well known. The screen grid 3 is shown connected through aresistance 23 to a suitable tap point of the high tension source I,which may be a battery as shown or a potentiometer resistance andby-passed by a condenser 24 as well understood by those skilled quenciesto be transmitted by the amplifier.

The input terminals a b are connected directly across the primary oftransformer 8, the secondary thereof being connected to the grid 2 andto the cathode of the valve 1 through grid biasing resistor!) in thecathode lead shunted by a condenser ID in a manner well known. The plate5 of the valve is connected through the primary of tuned or untunedinter-stage transformer I2 to theplate supply 1. The secondary of theinterstage transformer I2 is connected to the input grid I3 of asucceeding amplifier valve l4 through a coupling resistance l5 andbiasing resistance l6 shunted by condenser I'I. Valve I4 being similarto valve I of the pentode type has a screen grid I8, a suppressor gridI9, and a plate or anode 20. The anode 20 of this valve is connectedthrough the primary of the output transformer 22 to the plate supply I.The inter-stage transformer I2 in the example shown as well as theoutput transformer 22 are provided with condensers across theirprimaries and secondaries similarly to transformer 8 and are preferablytuned to the same frequency as the, latter or broadly to the frequenciesto be transmitted through the amplifier. The secondary of transformer 22is connected to the output terminals 0, d. The suppressor grid I9 ofvalve I4 is directly internally connected. with the cathode in a mannerwell known. The screen grid [8 is shown connected in a known mannerthrough a voltage drop resistance 23 to the high tension source 'I andis bypassedv to the cathode by a capacity 24. There is provided furthera shunting condenser 25, across the high tension source I. As isunderstood the tuning condensers for the transformers 8, l2 and 22 maybe omitted and the latter designed to be resonant to the desiredfrequency or band of frequencies by virtue of the inductances anddistributed capacities thereof.

Normally the amplifier as above described will amplify in a known mannerthe frequencies to which the transformers 8,. I2 and 22 are tuned whichwill be of a band pass character having sloping sides and which withouta'high regenerative effect in the amplifier itself will, as in the caseof an intermediate amplifier, be broader than the frequency band desiredin such a manner that interfering signals from adjacent carrier waveswill be transmitted and amplified. To eliminate such interferingsignals, there are provided, in accordance with the improvement of theinvention, filter networks 26 and 21 connected between the plate 5 ofvalve l and the input circuit of valve 14. As is known, the voltagevariations on the plate 5 of tube I and grid I3 of valve I4 are 180 outof phase with the input voltage impressed upon the grid 2 of tube I,provided the primary and secondary of transformer I2 are connected withthe proper polarity, or if a resistance-capacity coupling network ofknown construction is employed in place of the transformer I2. Thevoltage Variations derived in the example shown from the plate 5 areimpressed through the coupling condensers 28 and 28 upon the multiplenetworks 21 and 26. Network 26 is a low pass filter network which typeof network, as is known, is effective in preferredly passing currentshaving frequencies below a predetermined cut-off frequency and ofsuppressing currents having frequencies above said cut-off frequency.Network 21 is connected in parallel to network 26 in the same circuitand is of the high pass type effective in preferredly passing currentshaving frequencies above a predetermined cut-off frequency and ofsuppressing currents having frequencies below said cut-off'frequency.Networks 26 and 21 have been illustrated solely as one particular typeof network to perform the function desired. More complex or moresimplified filters of any desired construction may be employed in placeof those illustrated to suit any special requirement. Likewise, one halfsection terminating filters may be employed at the input as well as theoutput end of each of the filters with or without correspondingterminating resistance, so as to maintain the impedance of all circuitsto which they are connected constant.

The outputs of the low pass filter 26 and high pass filter 21, as shownare connected in parallel through a gain control resistance network andcoupled through coupling condenser 45 to the grid 36 of valve 31; thelatter grid being provided with a grid leak or coupling resistance 35.Valve 31 represents an amplifying valve of the pentode type similar tovalves I and I4 having a cathode, an input or control grid 36, a screengrid 46, a suppressor grid 41 and a plate or anode 40, the suppressorgrid 41 being directly internally connected with the cathode in a wellknown manner. The screen grid 46 is connected through a voltage dropresistance 48 to the high tension source I and shunted to ground orcathode by a condenser 49. The output of valve 31 is shown connectedfr'omthe plate or anode 40 throughcoupling resistance 42 to the hightension source 'I and through coupling condenser 43 to a point on thecoupling resistance I5 in the grid input circuit of valve I4. Thevoltages of the frequencies to be discriminated as selected by thefilters 26 and 21 being applied across coupling resistance I5 attests inthe input circuit of valve It, are 180 out-of phase'with the voltages ofthe same corresponding frequencies impressed upon this input circuitthrough the transformer I2, and therefore the voltages of thesefrequencies will be neutralized and will not be further transmitted oramplified.

There is thus formed one auxiliary feed forward circuit path from astage other than the last one in the propagation direction of theamplifier to any following stage in that direction,

. modifying ina desired manner the inputp'otenother type ofphase-shifting or phase-rotating 7 off at frequency f2.

device or circuit known in the art may be em-' ployed for this purposeand the phase rotation or correction may be also affected in the mainamplifier path for the signal currents passed through the latter such asin or preceding to the grid circuit of the valve l4.

Figure 2 shows the characteristic curves (output response in db. or anyother unit plotted against frequency) of the above described amplifiercircuit of Figure 1. The curve A therein represents the normallytransmitted frequency band through the amplifier without theneutralizing or balancing circuit according to the invention. As will benoted, this curve having sloping ascending and descending branchesextends beyond the desired frequency band f1-f2 and it is these portionsof the characteristic curve which effect or cause interfering signalssuch as in the case of an intermediate frequency amplifier as employedin superheterodyne receivers or the like. The curve B represents thecharacteristics of the low pass filter 26 of Figure 1 having a cut-offpoint at the frequency f1. Curve C represents the characteristic curveof the high pass filter 27 of Figure 1 having a out- It is apparenttherefore that the only frequencies which will be transmitted throughthese filter networks 26 and 21 will be the frequencies below f1 and thefrequencies above f2. Therefore the voltage derived from thesefrequencies above and below'the desired band f1 to f2, will oppose thesignal voltages of corresponding frequencies impressed upon the grid l3of valve M thereby causing neutralization thereof. As a result there isobtained in the output circuit or across the primary of transformer 22 acharacteristic with a sharper cut-off than would have been possiblewithout this novel arrangement.

The curve D in Figure 2b represents the resulting characteristic curveobtained by the use of the invention and as will be noted the slopes atthe lower and upper part of this curve are steeper than in the originalcurve A of Figure 2. Thus, by this arrangement the over-all selectivityof the amplifier is considerably increased resulting in the eliminationof interference from adjacent signal channels. As is understood from theforegoing the inventive circuit, among various other uses, has greatadvantages when embodied in the intermediate section of asuperheterodyne radio receiver in enabling a substantial increase ofreceiver selectivity without affecting the fidelity of reproduction. Thefilter 26' and 21 may be designed tohave the requisite critical orcut-off v frequencies f1-'f2, by proper dimensioning" or the capacityand in" ductance units in relation to each other and to the impedance ofthe associated input and output circuits connected to thei'nput andoutput terminals of the filters.

In the above description of the operation of Figure 2 and in thedescription of the circuits described hereafter, it is apparent that thevoltages derived from the parallel filter network or forward feedcircuit in the case of Figure 1 must, to suppress the undesiredfrequencies, have voltages exactly out of phase with the voltagesdeveloped in the grid circuit to which the output of the filter networkis connectedso as to obtain complete neutralization of theseundesired-frequencies. In other circuits wherein, instead ofneutralizing the undesired frequencies the filter networks form acorrecting or equalizer circuit for equalizing distortion of a line orthe like and where these voltages superimposed upon the voltages in thegrid circuit of the tube must ,be in phase, phase shifting devices suchas Wheatstone resistance capacity networks or other well known phaseshifting devices or systems may be employed in connection with thefilter or equalizer systems so as to obtain the proper phasing of thesuperimposed voltages. Likewise when employing transformers such as thetransformer l2 of Figure 1, reversal of phases may also be obtained byreversing the connections or the direction of winding of the primary orsecondary of the transformer.

Figure 3 is illustrative of another embodiment of the invention.According to this modification twin-triode valves are employed connectedin a cascade arrangement with one of the triode units in one of thevalves being employed as an amplifier for the currents passing throughthe wave discriminating networks. In the figure, numeral 50 representsan amplifier valve of the twin-triode type of standard constructionhaving a dual cathode, an input or control grid 5| and a plate or anode52, comprising one of the triode units, and the cathode further controlgrid 53 and the other plate or anode 54 comprising the other triode unitin this valve. A line or a preceding amplifier or transmission circuitis connected to the input terminals a, b and the signals therefrom areapplied through coupling condenser 3| to grid 5|, the latter beingprovided with its usual grid coupling resistance 32, and the well knownbiasing means, resistance 9 and condenser H). The plate 52 of thistriode unit is connected on the one hand through plate load resistance55 to the high tension source 1 and through a coupling condenser 56 tothe input circuit of the first triode unit of a second twin-triode valve51, the input circuit thereof comprising the grid 58 coupling resistance5 9 .and biasing resistance l6 shunted by condenser II. The output orplate 6| of this triode unit is connected through a known resistancecapacity coupling network comprised of resistances 62 and 63 and acapacity 64 to the input grid 65 of the second triode unitof this valve,the output or plate 66 of this triode unit being connected on the onehand through plate load resistor or impedance 34 to the high tensionsource 1 and through coupling condenser 33 to output terminal 0, theother output terminal at being connected to the common ground or zeropotential point of the system. The grid 53 of the second triode unit'ofvalve 50, which triode unit is used as the amplifier in the forward feedcirc'uit including the wave discriminating fitworks, derives" itspct-ennui -rrtm the input coupling resistance 32, which may beadjustable so as to vary the inputsignal level to be applied to thediscriminating networks. The output or plate 54 of this triode unit isshown connected to the filter or equalizer 61, through terminals e and jtothe high tension source I, the output. of filter Bl being'appliedthrough terminals g and h in series withresistance 63 to the inputcircuitof; the second triode unit of valve 51.

Thecascadeamplifier consisting of the three triode units that is theunits having input circuits connected togrids 5|, 58, 65, form a threestage resistance coupled amplifier having normally a fairly fiatfrequency characteristic depending upon the constants of the loadresistances and the coupling condensers 55, 62, 34 and 56, 64 and 33,respectively. To obtain a band pass frequency characteristic of, adesired band width from. such a resistance coupled amplifier, filternetworks-similar to the previously described low pass filter 28 and highpass filter 27, may be employed in the filter unit 61, provided with theusual load resistance for plate circuit 54 and coupling, condenser 28asshown in Figure 1. in such an arrangement as described, there may beimpressed upon the input circuit of grid 65 in series with theresistance, 63 voltage variations of the frequencies selected by thefilters, 180 out of the phase with the voltage variations of the samefrequencies impressed upon the input circuit from the line throughcouplingcondenser 64, thereby neutralizing or annulling these voltagevariations and producing in the output or plate circuit 65 amplifiedvoltage variations of the desired frequencies between f1 and f2, asshown in the curves of Figures 2a and 2b.

The arrangements described heretofore all relate to means for obtaininga band-pass frequency characteristic from broadly tuned or flatfrequency response amplifier circuits or for obtaining improved bandpass characteristics from band pass "characteristic amplifiers. It isapparent, that by selecting other types of filters such as band passfilters, 'as shown in Figure 8 later referred to, or'any other type ofequalizing or wave correcting network any desired frequency response,band pass characteristic or band elimination characteristic may beobtained.

Figure 4 represents a typical equalizer or wave correcting network whichmay be employed in connection with any of the previous described figuresand especially in connection with the circuit'of Figure 3. Normally,when the main amplifier portion of the circuit of the type shown inFigure 3, that is the three stage resistance coupled cascade amplifierisconnected to a transmission channel, such as a telephone or carriercurrent line, the distortion characteristic of the line will affectsignals transmitted by the line in such a manner as to attenuate certainfrequencies thereof, which frequency attenuations will be repeated bythe amplifier and will produce in the output ofthe' amplifier a responsecharacteristic similar to the distortion characteristic of the line.Methods have been employed in the past to equalize thesedistortioncharacteristics by employing equalizer networks in theamplifier proper, however such equalizers have the disadvantage ofintroducinglosses into the circuits thereby decreasing the overall gain'of the amplifier system. By the novel methoddisclosed such distortioncharacteristics of the transmission channel may bereadily compensated orneutralized by the employment of an equalizer or wave correcting networkwithout impairing the efiiciency of the amplifier system. Figure 5 showsa curve representing by way of example the transmission characteristicsof a transmission circuit or apparatus and the curve of an equalizeremployed in a neutralization circuit of the invention to obtain aresponse in the output circuit of the amplifier in which the distortioncharacteristics of the line are not repeated.

As shown in Figure 5, curve E illustrates the frequency response of acircuit or translating device connected to the input terminals a, b ofFi ure 3,through the usual coupling means. As will be noted, frequenciesbelow a have been attenuated, and frequencies above a are, therefore,impressed upon the input of the amplifier with increased amplitude overthe attenuated frequencies. In order to prevent such distortion theequalizing or wave correcting network as illustrated in Figure 4,connected to the usual plate load resistance 10, and coupling condenserH, should be so designed that the output response from the plate 66 ofvalve 51 corresponds to curve G of Figure 5. The equalizer as shown inFigure 4 consists of series elements, comprising a resistance l2,resonant circuit inductance 13 shunted by condenser 14, resistances l5and 16, shunted by condenser 11 and by inductance l8 and condenser 19 inseries therewith and a shunt element connected from the junction pointof the resistances l5 and I5, and consisting of inductance in serieswith a shunt resonance circuit comprising inductance BI and condenser82. The output of the equalizer includes a couplin resistance 83connected to the output terminals g, h, corresponding to the terminalsof equalizer 61 of Figure 3. -The constants of the component elements ofsuch an equalizer are well known and any desired equalization, orattenuation, for any desired frequency or frequency band, and having anydesired sharpness or cut-off, can be obtained byproper selection of thecomponent elements.

Curve F of Figure 5 shows the frequency response characteristics of theequalizer of Figure 4 and it will be noted that the frequencies below aare attenuated by the equalizer, whereas the frequencies above a aretransmitted with varying attenuation through the equalizer according toa characteristic similar to the response characteristic E. Therefore,the voltage variations derived across the coupling resistance 83 ofFigure 4, corresponding to the frequency response as represented bycurve F in Figure 5 when impressed upon the input circuit of the grid 65through terminals g; h of Figure 3, will equalize the correspondingvoltage variations of corresponding frequencies impressed upon the inputcircuit of grid 65 from the line through coupling condenser 64,resulting in a frequency response corresponding to curveG of Figure. 5,in the output circuit of plate 66 and across output terminals 0, d ofthe amplifier. It will be apparent that such equalizer networks, orfilters, should be designed to meet the specific problem involved in anyparticular case so as to produce in the output circuit of the amplifier,a predetermined frequency response.

Heretofore the circuits described have embodied the novel feature of theinvention of impressing upon an amplifier system at a point of higheramplification level variations corresponding to frequencies or frequencybands to be equalized derivedfrom a point of lower amplification leveland neutralizing or annulling. by opposing the corresponding frequencyvoltage variations impressed upon the amplifier from an input circuit orline. Another modification of the inven- 5 tion is to utilize thefrequencies to be transmitted through the amplifier and superimposingvoltages of the desired frequencies upon the amplifier to becomeadditive to the signal voltages in the amplifier, thereby to increasethe overall gain of the amplifier so as to obtain a band passcharacteristic or any desired shape or output frequency response.

Figure 6 is a modification employing this additive feature, wherein thevoltages of the predetermined frequency or frequencies passed by thefilters or equalizer are superimposed upon the signal voltages in phasetherewith, thereby producing a cumulative increase in the voltagesimpressed upon the grid for the. frequency or frequencies selected bythe filter or equalizer networks. In general, the amplifier proper ofFigure 6 is similar to the amplifier previously described in connectionwith Figure 3. The input a, b from a line or a preceding amplifier iscoupled through the input transformer 85 in the example shown aniron-core audio or low frequency transformer the "secondary of which isshunted by a coupling resistance 86, and connected to the input grid 5|of the first triode unit ofv thetwin-triode tube 50. Theoutput or plate52 thereof is coupled through, a transformer 9! to the grid 58 of thefirst triode unit of the second twin-triode 51, through a shuntresistance 8.! placed across the secondary of. the transformer 86, and agrid series resistance 88. The resistances 81 and 88 form. a constantimpedance input and output network. The output or plate 6| of thistriode unit is connected on the one hand through plate load impedance 89in the example shown an iron-core choke coil to the high tension sourceI and through coupling condenser 64 and grid leak resistance 63 to theinput grid 65 of the second triode unit, the output or plate 66 thereofbeing connected to the high tension source 1 through the plate loadresistance 34 and to the output terminal c through coupling condenser33. The signal energy for the filter or equalizer 61 is applied, fromthe coupling resistance 86 through series grid resistance 86' to thegrid 53 of. the second triode unit of the first twin-triode valve 50.The output or'plate 54 thereof, is connected to the filter or equalizernetwork 6-! through terminals e and J to the high tension source I, theoutput terminals g and h of the filter or equalizer network 61 beingconnected toa coupling resistor 90, the latter being in series with thegrid resistors 8! and 88 in the input circuit of grid 58 of the firsttriode unit of the valve 51. As will be apparent from the above, thevoltage derived across the coupling resistance 90 from the filter orequalizer networks 61 will be in phase with the voltage impressed uponthe grid 58, from the 'transformer 9|. Therefore the voltage varia- 65tions of the frequencies selected by the filter or equalizer network 61will become additive to the voltage variations of the same correspondingfrequencies repeated by the transformer 9| causing a cumulativeresultant thereof which is then ap- TO plied to. the grid 58, producingin the output or plate 6| an amplified response of these selectedfrequencies, which will be further repeated and amplified by thesucceeding amplifier.

To illustrate the operation, and performance, 75 of the circuit ofFigure 6, there may be employed inlthe filter or equalizer 61 anequalizer or wave corrective network similar to the type shown in Figure3 but having a characteristic response corresponding to curve H ofFigure 5, wherein it will be noted that the frequencies below a will be5 transmitted through the equalizer and the frequencies above a will beattenuated. The characteristics of the equalizer or wave correctingnetwork should correspond to the attenuated portion of the impressedinput signal as shown by 10 curve E in Figure 5. It is apparent, thatthese voltage variations of the frequencies transmitted through theequalizer or wave correcting network being impressed upon the grid input58 of Figure 6 in phase with the impressed voltage 15 variations of theinput signals impressed thereon from transformer 9| and of the sameapproximate ainplitude that the attenuated frequencies discriminated bythe line or amplifier, are equalized and the result will be a curve as20 represented by G in Figure 5. As explained in connection with Figures3, 4, and 5 previous y, other types of equalizer or filter networks maybe employed to obtain any predetermined 'fre-- quency response from theamplifier. 25

Figure 7 represents a modification of the circuit arrangement of Figure6. The input a, b is coupled through transformer 85 to the grid 5| ofthe first triode unit of valve 50, through impedance correcting gridresistance 88' and 3 coupling resistance 86 shunted across the secondary of the transformer. The output or anode 52' of this triode unit,is connected on the one hand through plate impedance 92 to the hightension source I and through coupling condenser 93 to the grid 58 of thesecond twin-triode valve 51. The grid 53 is provided with a grid leakresistance 59 in connection with the usual grid biasing resistance [6and by-pass condenser IT. The output or plate 6| of this triode unit isio coupled as previously described in connection with Figure 5, throughresistance 62 to the high tension source I and through couplingcondenser 64 to grid 65, the latter being provided with the couplingresistance 63. The output or plate 66, is connected'through the plateload resistance 34 to the high tension source 1 and through couplingcondenser 33 to the output terminal 0. The amplifier for the filter orequalizer network, in this case, is connected from the output or plate52 of the first triode amplifier unit in valve 50 through couplingcondenser 95 to thegrid 53 of the second triode unit in valve 5.0, thegrid 53 being provided with the usual resistance grid leak 96 andbiasing means 9 and Ill. The output 55 thereof or plate 54 is connectedto the filter or equalizer 61 through terminals 6 and ,f to the hightension source I. The output or terminals 9 and h of-the filter orequalizer are connected in series with the grid resistance 63 in theinput circuit of the last triode unit. It will be apparent from theabove description that the voltage variations derived from the output ofthe filter or equalizer will be in phase with the voltage variationsimpressed upon the grid 65 65 through the coupling condenser 64.Therefore the voltage variations of the predetermined frequency orfrequencies transmitted by the filter or equalizer will becomecumulative to the voltage variations of the same frequencies impressed.upon the grid 65 through coupling condenser 64, resulting in. producingin the output or plate circuit an increased voltage amplitude of thefrequencies selected by the filters or equalizers.

For equalizing or wave correction, equalizers.

or wave correction networks as previously described in connection withFigures 3, 4, 5, may be employed. For band pass characteristics or whereincreased amplitude of a predetermined frequency band is desired, bandpass filters may be employed in place of the equalizer or wavecorrecting networks.

Figures 8 and 9 are illustrative of band pass filter networks adapted totransmit frequencies of any desired band width dependent upon theconstants of the component elements thereof.

In Figure 8 there is shown a band pass filter which may be arranged inthe feed forward auxiliary circuit in any of the precedingillustrations. This filter proper shown at 98 derives its input from thecoupling resistance 99 connected across the terminals e and ,f, thefilter itself consisting of a number of sections and comprising asseries elements capacities I00 and IOI and shunt elements consisting ofan inductance I03 shunted by condenser I02. The output of filterterminates across the load or variable coupling resistance I04, in theoutput terminals g and h. This filter may be used in the circuit inFigure 7 in which case the voltage variations transmitted therethroughwill be applied in phase with the voltage variations impressed throughthe coupling condenser 64 upon the grid 65. It is obvious that wheresuch a band pass characteristic is desired that the coupling means inthe amplifier proper such as the input transformer 85, the intercouplingelements 92, 93, 59' and 62, 63, 64 maybe replaced with transformersbroadly tuned to the frequency band desired or the intercouplingelements may be similar band pass characteristic networks designed totransmit the same frequency band as the additive filter system. In thislatter case-the additive system will compensate for the attenuationlosses through the intercoupled networks and in turn will provideincreased selectivity in addition to this increased gain.

Figure9 represents another type of band-pass filter consisting of alow-pass filter network I05 in series with a high-pass filter networkI06, the filters obtaining their potential from coupling resistance 99connected to input terminals e and ,f and the output thereof beingconnected through the coupling or load resistance I04 to the outputterminals j and it. Each filter illustrated may consist of any number ofsimilar sections and each may be provided with input and output halfsection filters, and/or may be provided with input, intercoupling andoutput terminating resistance so designed as to maintain properimpedance characteristic for the adjacent circuits to which thesefilters are connected.

Figure 10a illustrates the characteristic curve of a typical filter suchas illustrated in Figure 9. As noted, the cut-off points of therespective low-pass and high-pass filters are overlapping, that is, thelow-pass filter I05 of Figure 9, as illustrated by curve J in Figure10a, is designed to have a cut-off at frequency f2 whereas the highpassfilter I06 of Figure 9 is designed to have a cut-off as illustrated bycurve K, at frequency f1. Figure 10b represents the transmission curve Lof the combined filters I05 and I06 of Figure 9. Curve L comprises afrequency band I) between frequencies f1 and f2 corresponding to thecross over cut-ofi points as represented by curves F and A of Figure10a, as the frequency band transmitted by the combined filters.

Figure 11 represents a further modification and improvement of theinvention and as illustrated, combines the two methods disclosed namelythe neutralizing and the additive or cumulative methods into a singlesystem toproduce a bandpass characteristic amplifier for any desiredband width having sharp or steep cut-off points at the upper and lowerextremities of the frequency band to be transmitted and in which nosacrifice of gain of the amplifier proper is made.

In Figure 11, the amplifier proper comprises three stages embodyingpentode valves I, I4 and I08. The input circuit of the valve I obtainsits signal voltage through input transformer B, the primary of which isconnected across input terminals a and b. The secondary of thetransformer is provided with a shunt coupling resistance 32 from whichthe input signal voltage for the filter network system is derived. Allof the other elements of the amplifier proper, have been previouslydescribed in connection with the other figures and therefore thedescription thereof will not be repeated. The neutralizing and additivesystem comprises the twin-triode valve IIO, the filters III, H2, H3 andH4 and associated circuits. The grid II5 of the valve N0 of standardconstruction, derives its input potential from the input couplingresistance 32.

For the additive system, that is, the system in which the selectedfrequency voltage variations are superimposed in phase on the signalvoltage variations, only this single triode unit is employed as theamplifier. The output or anode IIB thereof is connected through plateload resistance II! to the high tension source I, and to the band-passfilters I I3 and H4 through terminal e. The filter H3 is a low passfilter and I I4 is a high pass filter and both may be similar inconstruction to the band pass filters previously described in connectionwith Figures 8 and 9 and should be designed to pass the band offrequencies desired asfor example a band between frequency ,f1 to is asrepresented in Figure 101). It is apparent therefore, that the output ofthe filters connected to terminals g and It will furnish to the couplingresistance II8, voltage variations of the frequencies of the band I).The coupling resistance 8 as shown, is connected in series with thecoupling resistance 59 in the grid circuit of valve I4. The voltagevariations supplied by the band-pass filters to the coupling resistanceI I8 being in phase with the signal voltage variations impressed uponthe grid I3 of valve I4 through coupling condenser 93, will thereforebecome cumulative thereto.

' For the neutralizing system as shown in this Figur'e'll, the outputfrom the plate III; of the valve, I I0, is coupled to the grid I I9 ofthe second triode unit thereof, through coupling condenser I20, the gridII9 being provided with the usual grid leak or coupling resistance IZI,the output or plate I22 of this triode unit being coupled through plateload resistance I23 to the high tension source I and to the parallelconnected band discriminating networks III and H2. It will be apparent,thatthere will be supplied to these filter networks a greatly amplifiedsignal corresponding to the signal impressed upon the cascadeamplifierproper through the input coupling transformer 8. The filterdiscriminating networks III. and H2 correspond to the similar filterdiscriminating networks 26 and 21 described in connection with Figure 1,the low-pass filter II2, having a cut-off point at frequency f1, as

shown by the curve B in Figure 2a or at the same frequency at which thehigh-pass filter I I4 has its cut-off point as shown by curve K inFigure 10a, and the high pass filter III having its cut-off point atfrequency is as shown by curve C in Figure 2a, or at the same frequencyat which the low-pass filter H3 has its cut-off point as shown by curveJ in Figure 10a. It is apparent therefore, that the low pass filter H2and'high pass filter I II will pass all frequencies below frequency f1and all frequencies above frequency f2 thereby discriminating for theband of frequencies between f1 and is. The output of the low pass filterH2 coupled through the blocking condenser or coupling condenser I24, andthe output from the high pass filter III are connected to the couplingresistance I25 which latter is connected in series with the couplingresistance I26 of grid of the last pentode tube I08. The voltagevariations of the frequencies passed by the filters III and H2 will be180 out of phase with the signal voltage variations of the samefrequencies impressed upon the grid of the valve IE8 through thecoupling condenser 33 from the preceding amplifier valve I4. It isapparent, therefore, that the voltage variations of these frequencieswill be neutralized and will not be repeated through the pentodeamplifier valve I08.

Figure 12 shows curves illustrating the function of the circuit ofFigure 11. In Figure 12a curve M represents the typical responsecharacteristic of the three stage cascade amplifier and may alsorepresent the input signal impressed upon the input terminals a, bthereof. Without the novel features of the invention this input signalwould be repeated by the amplifier and would produce a similar amplifiedfrequency response in the output thereof. Curve N represents thetransmission characteristic of the low pass filter H3 having a cut-offat frequency is. Curve 0 represents the transmission characteristic ofthe high pass filter H4 having a cut-01f at frequency ii. In Figure 12bcurve P represents the transmission characteristic of the frequenciesbetween f1 and f2 passed through the filters H3 and H4, the voltagevariations of which are applied across the coupling resistance H8. CurveQ thereon represents the characteristic curve of the voltage variationsof the input signal applied through the coupling condenser 93 from thepreceding amplifier valve I upon the grid I3 of amplifier valve I l. Asthe voltage variations represented by curve P across the couplingresistance H8 are in phase with the input signal voltage variationsthere will be applied to the grid I3 the resultant or a cumulation ofthese voltage variations as shown by curve R in Figure 122), whichvoltage variations will then be repeated or amplified by the valve I4and acbordingly impressed on the grid of the succeeding valve I08,through coupling condenser 33. As only the voltage variations of thefrequencies in the band between frequency f1 and is are desired in theoutput of the amplifier proper, the voltage variationsof the frequenciesbelow frequency f1 and voltage variation above the frequency is, whichwould be repeated through the amplifier valve I08, as shown on the curveB of Figure 12d can be eliminated by the use of the neutralizing systemof Figure 11. As explained in the following Figure 120 S represents thetransmission characteristic of the low pass filter H2 having a cut-offat frequency f1 and curve T represents the transmission characteristicof the high pass filter I'II having cutoff atfrequency h; In Figure 12dcurves'S and T represent the voltage variations of the frequencies below11 and above f2 which are applied across the coupling resistance I25 andas previously explained as these voltage variations-are 180 out of phasewith the voltage variation applied to the grid of amplifier valve W8through coupling condenser 33 as shown by curve R, these impressedvoltage variations will neutralize or oppose the voltage variations ofthe frequencies below frequency f1 [and above frequency f2, resulting inimposing on the grid of valve I08 only the voltage variations betweenfrequencies 71 and is. These voltage variations will be repeated andamplified by valve I68 producing in the output or plate circuit thereofa, band pass characteristic corresponding to curve U in Figure 12e.

Many other modifications may be made to the circuit of Figure 11 such asemploying wave correcting networks without departing from the scope ofthe invention. I do not wish to be limited solely to the devicesillustrated as employed in connection with this invention.

It is understood that any other type of network, filters, equalizers,etc., for producing the effects such as for obtaining high pass, lowpass, band pass, equalization, band discrimination or wave correctionmay be employed with equal advantage and without departing from thespirit of the invention. It is further understood that the invention maybe equally employed for translating or amplifying both low frequency andhigh frequency signal waves such as audio or speech currents, as wellas, intermediate frequency, high frequency, or ultra-high frequencywaves or currents.

While I have shown particular combination of elements to illustrate waysof carrying out the invention, I want it clearly understood that any oneof these elements can be used by itself, or with any other element,besides those shown :and illustrated without departing from the scope ofthe invention.

It will further be evident from the above that the invention is notlimited to the specific arrangements of parts and. circuits shown andmethods disclosed herein for illustration, but that the novel thoughtand underlying principle of the invention, is susceptible of numerousvariations and modifications coming within the broad scope and spirit ofthe invention as defined in the appended claims.

The specification and drawings are accordingly intended to be regardedin an illustrative, rather than a limited sense.

I claim:

1. In the system for translating electric wave energy, an electron tubeamplifier comprising a plurality of amplifying stages in cascadenormally having a relatively broad input-output frequency responsecharacteristic, means for deriving wave energy from a stage ofrelatively lower order of said amplifier, a first band-pass typefrequency discriminating network having an input and an output andcutoff frequencies separated from each other by a predetermined rangecorresponding to the wave band to be transmitted by said system, meansfor impressing said derived energy upon the input of said network,further means for impressing wave energy from the output of said networkupon the input circuit of an amplifier stage of relatively higher orderin substantially like phase relation to the wave energy beingtransmitted through said input circuit, means for deriving further waveen- 12. In a system as claimed in the preceding claim comprisingamplifying means connected in the path of said hand-pass andband-elimination networks, and means for controlling the amplitude ofthe energies, impressed upon said amplifier from said networks to obtaina resultant bandpass type input-output frequency response characteristicof said system having a band width defined by the cutoff frequencies ofsaid first and second network.

PAUL J. LARSEN.

